In the manufacture of objects of silicon nitride by sintering powder of the material while using isostatic pressing, the powder is suitably preformed into a manageable powder body. Such preforming can be accomplished by subjecting the powder to an isostatic compaction, for example, the powder being in a sealed capsule of yielding material such as a plastic capsule. The compaction can advantageously be carried out without the use of a binder at a pressure of at least 100 MPa and at room temperature or any other temperature which is considerably below the temperature during the compression used in connection with sintering. The preformed product can thereafter be given its desired shape by machining. In preforming a body, it is also possible to use, among other things, conventional techniques for the manufacture of ceramic goods. In such techniques, the silicon nitride powder is usually mixed before preforming with a temporary binder, for example methyl cellulose, cellulose nitrate, an acrylate binder, a wax or a mixture of waxes and after preforming, the binder is driven off by heating so that the preformed powder body becomes essentially free from binder.
When the preformed powder body is subjected to the isostatic pressure at the sintering temperature, in order to give the desired dense sintered product, the body must be enclosed in a casing which, during the pressing, is able to prevent the pressure medium used, normally a gas, from penetrating into the powder body. The casing, which like its contents is liberated from undesirable gases during a processing stage prior to sealing, must of course also have sufficiently high strength or viscosity during the isostatic pressing so as not to itself penetrate into the pores of the preformed powder body. When a preformed capsule of glass is chosen as the casing, the glass must be of a high-melting type in order not to run off or penetrate into the powder body at the sintering temperature. However, such a glass cannot be prevented, when softening, from accumulating in pockets and other cavities of the preformed powder body. Such accumulations of glass often lead to fractures at protruding portions of the sintered object when the object cools down due to differences in the coefficients of thermal expansion between the silicon nitride and the glass. The above method is therefore only suitable for the manufacture of objects having a very simple shape.
If it is desired to manufacture objects having a very complicated shape, the casing can be formed by dipping the preformed powder body into a suspension of particles of high-melting glass or otherwise surround the body with a layer of particles of such glass and then heating the powder body under vacuum at a temperature such that the particles form a tight casing around the body. The latter method allows the application of a thin casing which closely follows the shape of the powder body so that accumulations of glass on the sintered object can be avoided and thus also the disadvantages connected therewith. A tight casing is only achieved at high temperatures since the glass of course has to be of high-melting type in order not to run off or penetrate into the powder body during the sintering of the silicon nitride.
To avoid dissociation of the silicon nitride due to the departure of nitrogen at these temperatures, proposals have been made to use a porous layer of glass of a low-melting type over a porous layer of glass of a high-melting type. In this known procedure, when the powder body in being degassed, the outer porous layer is transformed into a layer impermeable to the pressure medium. When a dense layer has been formed, pressure is applied on the enclosed powder body with argon or helium to counteract dissociation of the silicon nitride during the continued temperature increase. During this temperature increase, the glass in the outer layer reacts with the material in the inner porous layer while forming an increasingly high-melting glass and while still maintaining a layer impermeable to or impenetratable by the pressure medium. Finally a glass layer which is impermeable to the pressure medium is formed of the innermost part of the inner porous layer before the glass in the outer layer is able to run off. This last formed glass layer forms a tight casing around the powder body when the isotatic pressing of the preformed product is carried out at the sintering temperature.